Agar plate

Agar plate
Uses Microbiological culture
Related items Petri dish
Growth medium

An agar plate is a Petri dish that contains a growth medium (typically agar plus nutrients) used to culture microorganisms or small plants like the moss Physcomitrella patens.

Selective growth compounds may also be added to the media, such as antibiotics.[1]

Individual microorganisms placed on the plate will grow into individual colonies, each a clone genetically identical to the individual ancestor organism (except for the low, unavoidable rate of mutation). Thus, the plate can be used either to estimate the concentration of organisms in a liquid culture or a suitable dilution of that culture using a colony counter, or to generate genetically pure cultures from a mixed culture of genetically different organisms, using a technique known as "streaking". In this technique, a drop of the culture on the end of a thin, sterile loop of wire, sometimes known as an inoculator, is streaked across the surface of the agar leaving organisms behind, a higher number at the beginning of the streak and a lower number at the end. At some point during a successful "streak", the number of organisms deposited will be such that distinct individual colonies will grow in that area which may be removed for further culturing, using another sterile loop.[1]


In 1881, Fanny Hesse, who was working as a technician for her husband Walther Hesse in the laboratory of Robert Koch, suggested agar as an effective setting agent, since it had been commonplace in jam making for some time.[2]


An agar plate being viewed in an electronic colony counter
An agar culture of E. coli colonies

Like other growth media, the formulations of agar used in plates may be classified as either "defined" or "undefined"; a defined medium is synthesized from individual chemicals required by the organism so the exact molecular composition is known, whereas an undefined medium is made from natural products such as yeast extract, where the precise composition is unknown.[3]

Agar plates may be formulated as either permissive, with the intent of allowing the growth of whatever organisms are present, or restrictive or selective, with the intent of only allowing growth a particular subset of those organisms.[4] This may take the form of a nutritional requirement, for instance providing a particular compound such as lactose as the only source of carbon and thereby selecting only organisms which can metabolize that compound, or by including a particular antibiotic or other substance to select only organisms which are resistant to that substance. This correlates to some degree with defined and undefined media; undefined media, made from natural products and containing an unknown combination of very many organic molecules, is typically more permissive in terms of supplying the needs of a wider variety of organisms, while defined media can be precisely tailored to select organisms with specific properties.

Agar plates may also be indicator plates, in which the organisms are not selected on the basis of growth, but are instead distinguished by a color change in some colonies, typically caused by the action of an enzyme on some compound added to the medium.

Some commonly used agar plate types are:

Red blood cells on an agar plate are used to diagnose infection. On the left is a positive Staphylococcus infection, on the right a positive Streptococcus culture.

Blood agar

Hemolyses of Streptococcus spp. (left) α-hemolysis (S. mitis); (middle) β-hemolysis (S. pyogenes); (right) γ-hemolysis (= nonhemolytic, S. salivarius)

Blood agar plate

Blood agar plates (BAPs) contain mammalian blood (usually sheep or horse), typically at a concentration of 5–10%. BAPs are enriched, differential media used to isolate fastidious organisms and detect hemolytic activity. β-Hemolytic activity will show lysis and complete digestion of red blood cell contents surrounding a colony. Examples include Streptococcus haemolyticus. α-Hemolysis will only cause partial lysis of the red blood cells (the cell membrane is left intact) and will appear green or brown, due to the conversion of hemoglobin to methemoglobin. An example of this would be Streptococcus viridans. γ-Hemolysis (or nonhemolytic) is the term referring to a lack of hemolytic activity.[5] BAPs also contain meat extract, tryptone, sodium chloride, and agar.

Chocolate agar

Chocolate agar is a type of blood agar plate in which the blood cells have been lysed by heating the cells to 56°C. It is used for growing fastidious respiratory bacteria, such as Haemophilus influenzae. No chocolate is actually contained in the plate; it is named for the coloration only.

Horse blood agar

Horse blood agar is a type of blood-enriched microbiological culture media. As it is enriched, it allows the growth of certain fastidious bacteria, and allows indication of haemolytic activity in these bacterial cultures.

Thayer-Martin agar

Thayer-Martin agar is a chocolate agar designed to isolate Neisseria gonorrhoeae.

Thiosulfate-citrate-bile salts-sucrose agar

Thiosulfate-citrate-bile salts-sucrose agar enhances growth of Vibrio spp., including Vibrio cholerae.[6]

General bacterial media

Four types of agar plate demonstrating differential growth depending on bacterial metabolism
Fungi (ascomycetes) growing in axenic cultures, each of which is a culture of one selected organism and is free of all other organisms, enabling study of the cultured organism in isolation
Aspergillus niger growing in potato dextrose agar

Fungal media

Bottom view of a Sabouraud agar plate with a colony of Trichophyton rubrum var. rodhaini

Moss media

See also

Different specific types of agar:


  1. 1 2 Madigan M, Martinko J, eds. (2005). Brock Biology of Microorganisms (11th ed.). Prentice Hall. ISBN 0-13-144329-1.
  2. "History of the agar plate". Laboratory News. Archived from the original on 11 February 2010. Retrieved 2010-02-22.
  3. Baron S; et al., eds. (1996). Baron's Medical Microbiology (4th ed.). University of Texas Medical Branch. (via NCBI Bookshelf) ISBN 0-9631172-1-1.
  4. Ryan KJ; Ray CG, eds. (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. ISBN 0-8385-8529-9.
  5. "Archived copy". Archived from the original on 1970-01-01. Retrieved 2014-10-28.
  6. 1 2 Fisher, Bruce; Harvey, Richard P.; Champe, Pamela C. Lippincott's Illustrated Reviews: Microbiology (Lippincott's Illustrated Reviews Series). Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-8215-5.
  7. Ralf Reski and Wolfgang O. Abel (1985): Induction of budding on chloronemata and caulonemata of the moss, Physcomitrella patens, using isopentenyladenine. Planta 165, 354–358.

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